linux-kernel - Re: fastboot/async and initramfs: How am I supposed to know when devices are finished initializing?

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Message-ID: <ac3eb2510903291036w13dc34cbi97525e6dd9af1da5@mail.gmail.com>
Date:	Sun, 29 Mar 2009 19:36:59 +0200
From:	Kay Sievers <kay.sievers@...y.org>
To:	Thomas Bächler <thomas@...hlinux.org>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: fastboot/async and initramfs: How am I supposed to know when 
	devices are finished initializing?

On Sun, Mar 29, 2009 at 18:39, Thomas Bächler <thomas@...hlinux.org> wrote:
> Arjan van de Ven schrieb:
>>>
>>> Among other things, this loads the ata_piix on my machine. On older
>>> kernels I could assume that after these commands, /dev/sda* existed
>>
>> not if you have CONFIG_SCSI_SCAN_ASYNC set though..
>>
>>> and I could immediately access them (in my case, I run cryptsetup on
>>> /dev/sda6). But now, the devices don't exist here, but are only
>>> created a second or so later. As a result, the initramfs script has
>>> already bailed out as it couldn't find /dev/sda6 and assumed that the
>>> hard drive didn't exist.
>>
>> the CONFIG_SCSI_WAIT_SCAN method (basically loading that module to wait
>> for the scans to finish) will work for you......
>
> Thanks, I will integrate that into our initramfs. Will that also work for
> USB mass storage (which was already problematic with older kernels, you load
> the module you don't know how long it takes until the sdX devices are
> created)?

USB is interrupt driven, there is and will never be such a thing as
"scanned" or "settled" -- anything can come and go at any time.

In initramfs, you have to wait until the device shows up, not for a
random module to initialize, or a bus to be scanned -- that can never
work correctly, it's pure luck, that your logic was always slower than
the kernel.

You need a block device -- so you should just wait for the block
device, instead of making assumptions about initialization of drivers
or buses. :)

Thanks,
Kay
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